Abrasive compounds for a glass substrate for an optical disk platter and a magnetic disk platter is an aqueous dispersion of abrasive grain containing cerium oxide obtained by calcinating and dry milling bastnaesite ore or rare earth chloride. The abrasive grain is relatively inexpensive. However, its cerium oxide content is at most 50 to 90% and it is difficult to increase the purity any more since natural ore is used as a raw material. The average secondary particle size of the powder is 1 to 3 μm. It is difficult to decrease the average secondary particle size to 1 μm or less when the powder is made finer by a breakdown method such as dry milling.
Regarding the abrasive compound for SiO2 oxide film of a semiconductor device, Japanese Patent Application Laid-open No. Hei 5-326469 discloses an abrasive compound comprising cerium oxide having an average particle size of 1 μm or less and a cerium oxide purity of 99.5% or more. Japanese Patent No. 2864451 discloses that polishing with an abrasive compound having an average particle size of 0.1 μm or less and a cerium oxide purity of 99.5% or more gives rise to an oxide film of high quality.
On the other hand, in polishing a glass substrate for an optical disk platter and a magnetic disk platter, Japanese Patent Application Laid-open No. Hei 11-60282 discloses a method for polishing a glass substrate for a magnetic disk platter in which the cerium oxide content in an abrasive liquid is set to 0.5 to 8 wt %. International Publication No. WO98/21289 discloses a method for using an abrasive material having an average particle size of primary particle is 0.002 to 3 μm in an abrasive composition for a substrate for a magnetic recording medium comprising an abrasive compound, an abrasive auxiliary compound and water.
In recent years, the performance of a glass substrate for an optical disk platter and a magnetic disk platter tends to be higher in density and speed and therefore a polished surface of high quality having smaller surface roughness and average waviness is desired. However, it becomes increasingly difficult to obtain a good polished surface by use of cerium(IV) oxide abrasive grains having a cerium oxide purity of 50 to 90% and an average secondary particle size of 1 to 3 μm obtained by calcinating and dry milling bastnaesite ore or rare earth chloride.
It has been found out that in order to solve the problems, a abrasive compound for a glass substrate for an optical disk platter or a magnetic disk platter that can give a polished surface of high quality and that allows high speed polishing can be obtained by setting an average secondary particle size of cerium(IV) oxide particles to 1 μm or less and increasing the cerium amount for 95% or more in terms of oxide of the total amount of rare earth elements in the abrasive in order to supplement a decrease in polishing speed as a result of a reduction in the average secondary particle size of cerium(IV) oxide particle. The present invention has been achieved based on this discovery.